The present specification may employ the following abbreviations.
GAG: glycosaminoglycan
PG: proteoglycan
GlcNAc: N-acetyl glucosamine
GlcNS: N-sulfated glucosamine
GlcN: glucosamine
GlcA: glucuronic acid
IdoA: iduronic acid
IdoA(2S): 2-O-sulfated iduronic acid
HexA: hexuronic acid
aMan: 2,5-anhydromannose
AS: acharan sulfate
ACH: acharan (2-O-desulfated AS)
NAH: N-acetyl heparosan
NSH: N-deacetylated N-sulfated heparosan
HEP: heparin
CDSNAc-HEP: completely desulfated N-acetylated heparin
CDSNS-HEP: completely desulfated N-resulfated heparin
HS: heparan sulfate
C5-epi: heparosan-N-sulfate-glucuronate 5-epimerase
HG-5epi: heparosan-glucuronate 5-epimerase
It is known that acharan sulfate (AS) is a type of glycosaminoglycan (GAG), which is isolated from Achatina fulica and is known to be a linear GAG having a structure in which disaccharides formed of GlcNAc and IdoA(2S) (-[4GlcNAcα1-4IdoA(2S)α1]-), represented by the following structural formula (1), are repeatedly polymerized (Non-Patent Documents 1 and 2). Also, AS is known to have physiological activities similar to those of heparin (HEP) and heparan sulfate (HS), which are GAG having the carbohydrate backbone common to AS. Specifically, such activities include angiogenic inhibitory activity (Non-Patent Document 3), immunostimulatory activity (Non-Patent Document 4), hypoglycemic activity (Non-Patent Document 4), anti-coagulation activity (Non-Patent Document 5), anti-tumor activity (Non-Patent Document 6), and adhesion inhibitory activity against Helicobacter pylori (Non-Patent Document 7).

Biosynthesis of HEP and HS has been elucidated through analyses of carbohydrate structures provided by knock-out mice or mutant cells, and experiments evaluating substrate specificity of enzymes. The elucidated mechanism of the biosynthesis includes synthesis of N-acetyl heparosan (NAH), which is a GAG having a structure in which disaccharide units being formed of GlcNAc and GlcA (-[4GlcNAcα1-4GlcAβ1]-) are repeatedly polymerized; N-deacetylation and N-sulfation; epimerization of GlcA residues to IdoA residues; and O-sulfation, in this order (Non-Patent Documents 8 and 9). Among these steps, epimerization of GlcA residues to IdoA residues is catalyzed by a heparosan-N-sulfate-glucuronate 5-epimerase (C5-epi, EC: 5.1.3.17). Thus, all the known members of C5-epi involved in biosynthesis of HEP and HS are known to act on, as a substrate, N-deacetylated N-sulfated heparosan (NSH), which is a GAG generated through N-deacetylation and N-sulfation of NAH, and to not act on NAH itself (Non-Patent Documents 10 to 14 and Patent Documents 1 and 2).
Regarding Achatina fulica, proteome analysis has been carried out to collectively identify a group of enzymes involved in biosynthesis of proteoglycan (PG). The analysis has revealed a variety of enzymes involved in biosynthesis of PG core protein or GAG, based on the homology to a known protein (Non-Patent Document 15). Regarding synthetic pathway of HEP and HS, proteins, which are thought to serve as NAH synthase, N-deacetylase/N-sulfotransferase, and O-sulfotransferase have been found. However, no protein having a homology to C5-epi, which is a known epimerase, has been identified. In addition, regarding Achatina fulica, the enzymatic cascade in biosynthesis of AS has not been elucidated. Thus, the substrate on which an IdoA synthase acts and the product obtainable by the synthase reaction have not been identified.